This invention relates generally to high pressure metal vapor, such as sodium, lamp systems and more particularly is concerned with a circuit in which means are provided for interrupting power to the ballast of the system if the system should be energized while there is no lamp connected therein or if the high pressure metal vapor lamp is or becomes inoperative. Although not limited thereto, the invention will be described as applied to sodium vapor lamps.
The conventional high pressure sodium vapor (hereinafter HPSV) lamp system comprises a ballast whose input is connected to a relatively low voltage source of a.c. power and whose output has the HPSV lamp connected therein. The usual line furnished by the power company is at 120 volts. The ballast is required to perform several functions which comprise stepping up the line voltage to a voltage which will keep the HPSV lamp ignited; limiting the electric current which flows through the lamp once it has ignited; and furnishing the high voltage in the form of pulses required to ignite the lamp in the first place.
A metal vapor lamp is a gaseous discharge device and hence it requires a voltage much higher than its operating voltage to ignite it. In the case of the HPSV lamp this voltage is much greater than the operating voltage and may be as much as fifty times that voltage. This type of ballast typically includes a pulsing circuit which repeatedly applies high voltage pulses to the lamp until the lamp ignites and the flow of current causes the pulsing to stop. There are many variations of these pulsing circuits, and they are not intended to be continually energized; hence the components thereof are not built to carry high currents or to be subjected to the stress of continuous high voltage pulsing. These pulsing circuits are represented in the prior art by such patents as Attewell U.S. Pat. No. 3,407,334; Nuckolls U.S. Pat. No. 3,917,976; Nuckolls U.S. Pat. No. 3,963,958; and many others in related classes.
As in the case of all gaseous discharge lamps, since the lamp is a negative resistance device, as soon as ignited the lamp impedance drops drastically since ignition actually comprises ionization of the gas within the lamp envelope. The ballast of the system provides impedance as soon as current flows to limit the current, as for example, by utilization of a high leakage reactance transformer as one of the components. Ballasts for relatively low voltage lamps, such as for example fluorescent types, normally have no special pulsing circuits for ignition since the ratio of igniting to operating voltage is usually of the order of two to one.
In the case of the HPSV lamp system, when electrical power is applied to the input circuit of the ballast, the pulsing section commences to operate. High voltage pulses are applied to the HPSV lamp and in a few seconds when the lamp is cold, the lamp ignites and operating current flows through the ballast. By suitable self-operating switching means, the pulsing section of the ballast ceases operating and the current then flows through the portions of the ballast which are designed to carry this current normally.
In the event that electrical power is interrupted momentarily and then reapplied to the ballast input, as for example by a line failure or switching disconnection, the pulsing device again commences to operate to attempt to re-ignite the lamp. If the lamp is hot as is usual with HPSV lamps, the lamp will typically re-ignite in about a minute.
Normal use of a ballast in an HPSV lamp system will ensure ballast life of several years for a good quality product. The cost of HPSV ballasts which include the pulsing circuits described is substantially higher than the relatively simple ballasts of the type used for fluorescent lamp systems. Accordingly, any conditions which tend to decrease the life of a ballast for an HPSV lamp system must be avoided for economy as well as safety.
The principal problem with these systems is that the failureof the HPSV lamp to ignite for any reason keeps the pulsing circuit operating. The pulsing circuit or igniter, as it is often called, is not built for this type of continuous operation. Thus, since it is sold with the ballast, failure of the igniter results in loss of the entire ballast. This is true notwithstanding the fact that there is no failure of any other part of the ballast since there is no current flowing therein. As a matter of fact, the failure of fluorescent lamps in a typical system has no effect on most of the types of ballasts used therewith. An HPSV lamp ballast connected to a lamp that is not ignited or capable of being ignited cannot be expected to last for more than a couple of months.
Manufacturers of HPSV lighting equipment have attempted to obviate the loss of ballasts by educating their users to replace the HPSV lamps as soon as any failure is noted. In many instances, however, the physical location of the lamps or the expense of labor for replacement may result in delay which can be long enough to destroy the igniter or reduce its life materially. The ballast must be repaired or discarded when its igniter fails. It will remain inoperative until then.
According to the invention a circuit is proposed which includes means for sensing the flow of current through the HPSV lamp or the absence thereof to keep the ballast energized or to de-energize the same, respectively. By time delay means the normal time that is required for the HPSV lamp to ignite when hot is compensated for.
The following prior art is of interest:
Feinberg et al. U.S. Pat. No. 3,287,599;
Lee U.S. Pat. No. 3,721,832;
Rosen U.S. Pat. No. 3,890,545.
The Feinberg patent discloses a fault indicating circuit for a fluorescent lamp system which is equipped with an automatic resetting thermostat. Lee discloses a timing circuit for use with pumps, air conditioners and the like. Rosen describes a travelling wave tube protection circuit. None of these is believed to disclose the claimed invention herein.